TWI396772B - Alloyed hot dip galvanized steel sheet and producing method therefor - Google Patents

Description

Alloyed hot-dip galvanized steel sheet and manufacturing method thereof Technical field

The present invention relates to an alloyed hot-dip galvanized steel sheet which can be press-formed into automobiles, home electric appliances, building materials and the like, and a method for producing the same, and particularly, a non-uniform appearance, slidability (flaking resistance), and powdering resistance An alloyed hot-dip galvanized steel sheet excellent in chemical conversion treatability and a method for producing the same.

The invention of the present invention is based on the priority of the Japanese Patent Application No. 2009-023603, which was filed on February 4, 2009 in Japan, and the Japanese Patent Application No. 2009-022920, which was filed on February 3, 2009 in Japan. Etc.

Background technique

Since the alloyed hot-dip galvanized steel sheet has excellent weldability and coating properties compared with the galvanized steel sheet, it is mainly used in automotive body applications, and is widely used in a wide range of applications such as home appliances and building materials.

In the alloyed hot-dip galvanized steel sheet, the steel sheet is subjected to hot-dip galvanizing, and then heat treatment is performed to diffuse Fe in the steel and Zn in the plating layer to form an alloy, whereby the Fe-Zn alloy layer is formed on the surface of the steel sheet. It is said that the alloying reaction will preferentially occur from the grain boundary of steel. However, when the grain boundary contains more elements that are easily segregated, the interdiffusion of Fe and Zn will be partially blocked. Therefore, the alloying reaction becomes uneven, resulting in a difference in thickness. Due to the difference in the thickness of the plating layer, linear spots are generated, and the appearance is mottled and the quality is poor. In particular, in recent years, steel sheets have advanced toward high strength, and in steel sheets containing a large amount of elements such as P which are easy to segregate at grain boundaries, there is a problem that mottle is liable to occur. This problem is caused by the uneven concentration of P on the surface of the steel sheet and the grain boundary during heating of the steel sheet. In the concentrated portion of P, the interdiffusion of Fe and Zn during the alloying of the plating layer is hindered. Therefore, the alloying reaction between Fe and Zn causes a local difference in speed, and a difference in plating thickness occurs. In addition, as a strengthening method for steel, Si and Mn are inexpensively added. However, if the Si content in the steel exceeds 0.3% by mass%, the wettability of the plating will be greatly lowered. Therefore, there is a problem that plating failure occurs and appearance quality is deteriorated.

Therefore, various discussions have been made on alloyed hot-dip galvanized steel sheets having excellent appearance quality. For example, a method of grinding a surface of a plated steel sheet to have an average thickness Ra of a center line Ra of 0.3 to 0.6 and immersing in a hot-dip galvanizing bath to form an alloyed hot-dip galvanized steel sheet is known (for example, a reference patent) Document 1); and a method of forming a metal coating layer of Fe, Ni, Co, Cu, or the like before the hot-dip galvanizing of the steel sheet to be annealed (for example, refer to Patent Document 2). However, in these methods, it is necessary to have a process before the hot-dip galvanizing, and in addition to increasing the number of processes, there is a problem that the cost increases as the equipment increases.

Further, the alloyed hot-dip galvanized steel sheet is generally subjected to pressure forming and then used. However, the alloyed hot-dip galvanized steel sheet has a disadvantage of poor press formability as compared with the cold rolled steel sheet.

The reason for such poor press formability is due to the structure of the alloyed hot-dip galvanized layer. That is, in general, the Zn-Fe alloy plating layer produced by the alloying reaction in which Fe in the steel sheet is diffused into Zn in the plating layer is a coating layer composed of a Γ phase, a δ ₁ phase, and a ζ phase. As the Fe concentration decreases, the coating layer will sequentially decrease in hardness and melting point in the order of Γ phase, δ ₁ phase, and ζ phase. That is, the plated area (coated steel plate interface) that is in contact with the surface of the steel sheet produces a hard and brittle Γ phase, and the upper part of the plating layer forms a soft ζ phase. The ζ phase is soft and easily condenses with the pressurizing mold, and the friction coefficient is high and the slidability is poor. When the severe pressure forming is performed, the plating layer is condensed on the film and peeled off (flaking). . On the other hand, since the ruthenium phase is hard and brittle, it causes the plating layer to become powdery and peeling (powdering) during press molding.

When the alloyed hot-dip galvanized steel sheet is press-formed, it is important to have good slidability. Therefore, from the viewpoint of slidability, it is effective to form a high Fe alloy film having a high hardness and a high melting point and which is less likely to be condensed by high alloying of the plating film. However, such alloyed hot-dip galvanized steel sheets cause powdering.

On the other hand, from the viewpoint of powdering resistance, it is effective to form a low-Fe concentration plating film which suppresses the formation of a ruthenium phase in order to prevent the film from being low-alloyed in order to prevent pulverization. However, such an alloyed hot-dip galvanized steel sheet has poor slidability and causes peeling.

Therefore, in order to improve the press formability of the alloyed hot-dip galvanized steel sheet, it is required to achieve the opposite properties of the slidability and the powdering resistance.

Heretofore, as a technique for improving the press formability of the alloyed hot-dip galvanized steel sheet, a method of producing a δ ₁ main body alloyed hot-dip galvanized steel sheet has been proposed, which is in a high Al bath, and the Al concentration has been specified. In the alloying furnace of the high-frequency induction heating method, the alloying treatment is performed at a high intrusion temperature of the relationship, and then the temperature of the delivery side plate is 495 to 520 ° C (for example, refer to Patent Document 3). . Furthermore, a method for producing an alloyed hot-dip galvanized steel sheet has been proposed, which is maintained at a temperature of 460 to 530 ° C for 2 to 120 seconds immediately after the hot-dip galvanizing, and then cooled at 5 ° C / sec or more. The temperature is cooled to 250 ° C or lower to form a δ ₁ single-phase alloyed plating layer (for example, refer to Patent Document 4). Further, a method for producing an alloyed hot-dip galvanized steel sheet which is based on heating and cooling in the alloying treatment in the production of the alloyed hot-dip galvanized steel sheet in order to achieve both surface slidability and powdering resistance is proposed. The temperature (T) is multiplied by the time (t) and multiplied by the calculated temperature distribution to determine the temperature mode of the alloying treatment (for example, refer to Patent Document 5).

In the conventional techniques, the degree of alloying is controlled to achieve hardening of the alloyed hot-dip galvanized layer, and it is attempted to achieve both powdering resistance and peeling resistance which are disadvantageous in the alloying of the hot-dip galvanized steel sheet.

In addition, since the flat surface portion has a great influence on the slidability, a method for forming an alloyed hot-dip galvanized steel sheet is proposed, which is controlled by the flat portion of the surface, even if the surface layer contains more ζ phase coating. It can have good powdering resistance and slidability (for example, refer to Patent Document 6).

This technique is a method for producing a alloyed hot-dip galvanized steel sheet having excellent powdering resistance and excellent slidability, and has a coating film having a plurality of ζ phases in the surface layer by reducing the degree of alloying. However, the alloyed hot-dip galvanized steel sheet is considered to be required to further improve the peeling resistance (sliding resistance).

Further, as a method of improving the press formability of a zinc-based plated steel sheet, a method of applying a high-viscosity lubricating oil has been widely used today. However, due to the high viscosity of the lubricating oil, there are problems such as coating defects caused by poor degreasing during the coating process or unsettled pressurization performance due to exhaustion of oil during pressurization.

Therefore, a method of forming an oxide film mainly composed of ZnO on the surface of a zinc-based plated steel sheet (for example, refer to Patent Document 7) and a method of forming an oxide film of Ni oxide have been proposed (for example, refer to Patent Document 8). However, such oxide films have a problem of poor chemical conversion treatability. Then, a film in which a Mn-based oxide film is formed as a film for improving chemical conversion treatability has been proposed (for example, see Patent Document 9). However, the technique of forming such an oxide-based film has not specifically examined the relationship between the oxide-based film and the alloyed hot-dip galvanized film.

Advanced technical literature Patent literature

[Patent Document 1] Japanese Patent Laid-Open Publication No. 2004-169160

[Patent Document 2] Japanese Patent Laid-Open No. Hei 6-88187

[Patent Document 3] Japanese Patent Laid-Open Publication No. H9-165662

[Patent Document 4] Japanese Patent Laid-Open Publication No. 2007-131910

[Patent Document 5] Japanese Patent Laid-Open Publication No. 2005-54199

[Patent Document 6] Japanese Patent Laid-Open Publication No. 2005-48198

[Patent Document 7] Japanese Patent Laid-Open No. 53-60332

[Patent Document 8] Japanese Patent Laid-Open No. 3-119093

[Patent Document 9] Japanese Patent Laid-Open No. 3-249182

As described above, the alloyed hot-dip galvanized steel sheet is required to have good chemical conversion treatability (corrosion resistance). In addition, it is required to have a good surface appearance and good powdering resistance and slidability in a press forming process.

In view of such a situation, the present invention provides an alloyed hot-dip galvanized steel sheet and a method for producing the same, which are related to surface slidability (flaking resistance), powdering resistance, and radio plaque during press forming. The appearance is mottled, the surface appearance is good, and the chemical conversion treatability is excellent. In particular, an alloyed hot-dip galvanized steel sheet and a method for producing the same are provided, and an alloyed hot-dip galvanized steel sheet having a low heating rate and a low alloying treatment and excellent powdering resistance is provided with excellent surface slidability. Surface appearance and chemical conversion treatability.

During the alloying process of alloying hot-dip galvanizing, the appearance of mottledness causes poor quality due to the linear plaque caused by the difference in plating thickness. That is, in the portion where the alloying is faster, the alloy layer grows thicker than the surrounding portion, so that a line called a linear spot occurs. The inventors of the present invention have intensively studied the mechanism of the difference in the thickness of the plating layer, and have known that the galvanized layer is heated at a low speed to be alloyed, whereby the occurrence of the grain can be suppressed, and an alloyed hot-dip galvanized steel sheet excellent in appearance can be obtained. Further, in terms of press formability, when hot-dip galvanizing is performed, a large amount of Γ phase is formed. Therefore, the surface slidability (flaking resistance) at the time of press molding is good, but the powdering resistance is lowered. On the other hand, when the hot-dip galvanizing is performed as a low-alloying treatment, the formation of the Γ phase is reduced and the ζ phase is increased. Therefore, the powdering resistance at the time of press molding is good, but the surface slidability (flaking resistance) is lowered. Further, in the alloyed hot-dip galvanized steel sheet, formation of a ruthenium phase cannot be avoided. Then, the present inventors focused on an alloyed hot-dip galvanized steel sheet having excellent powdering resistance and low alloying degree, and a method for improving the disadvantage (surface slidability) thereof was carefully examined. As a result, it has been found that by forming a Mn-P-based oxide film on the surface of the alloyed hot-dip galvanized steel sheet having a low alloying degree, the disadvantage (surface slidability) of the alloyed hot-dip galvanized steel sheet having a low alloying degree can be remarkably improved. Both powdering resistance and peeling resistance can be considered.

The present invention has been completed based on the above findings, and the gist of the invention is as follows.

(1) An alloyed hot-dip galvanized steel sheet comprising a steel sheet, an alloyed hot-dip galvanized layer, and a Mn-P-based oxide film, wherein the steel sheet has C, Si, Mn, P, Al, residual Fe and is inevitable The composition of the impurities; the Zn-Fe alloy phase in the alloyed hot-dip galvanized layer is divided by the diffraction intensity Γ(2.59Å) of the Γ phase in the X-ray diffraction d=2.59Å lattice spacing d = δ 2.13Å _phase diffraction intensity of the δ ₁ (2.13Å) is 0.1 or less; lattice spacing d = ζ 1.26Å phase diffraction intensity of the ζ (1.26Å) divided by the lattice The δ ₁ phase diffraction intensity δ ₁ (2.13 Å) of the surface spacing d=2.13 Å is 0.1 or more and 0.4 or less; and the Mn-P based oxide film is Mn of 5 to 100 mg/m ² , P The surface of the alloyed hot-dip galvanized layer is coated in an amount of from 3 to 500 mg/m ² .

(2) The steel sheet may contain, by mass%, C: 0.0001 to 0.3%, Si: 0.01 to 4%, Mn: 0.01 to 2%, P: 0.002 to 0.2%, and Al: 0.0001 to 4%.

(3) The aforementioned diffraction strength Γ (2.59 Å) of the foregoing Γ phase of the aforementioned Zn-Fe alloy phase in the alloyed hot-dip galvanized layer in the X-ray diffraction may be 100 (cps) Hereinafter, the aforementioned diffraction intensity ζ (1.26 Å) of the ζ phase of the lattice plane spacing d = 1.26 Å may be 100 (cps) or more and 300 (cps) or less.

(4) The Fe content in the Zn-Fe alloy phase in the alloyed hot-dip galvanizing layer may be 9.0 to 10.5%.

(5) A method for producing an alloyed hot-dip galvanized steel sheet, wherein the steel sheet is subjected to hot-dip galvanizing, heated in a heating furnace, and after the temperature of the steel sheet on the side of the heating furnace reaches the highest reaching temperature, the furnace is subjected to a heat-retaining furnace. Cold alloying treatment to form an alloyed hot-dip galvanized layer, and a Mn-P-based oxide film containing Mn and P is formed on the surface of the alloyed hot-dip galvanized layer; the alloying treatment is 420 (° C.) T0, the steel plate temperature (°C) on the feeding side of the heating furnace is T11, the steel plate temperature (°C) on the inlet side of the cooling zone of the heat retaining furnace is T12, and the steel plate temperature (°C) on the sending side of the cooling zone is T21, the aforementioned heat retaining furnace The steel sheet temperature (°C) on the delivery side is T22, and the treatment time (sec) from the T0 to the heating furnace delivery side is t1, and the processing time from the heating furnace delivery side to the inlet side of the cooling belt of the heat retention furnace (sec) And t2, the processing time (sec) from the inlet side of the cooling belt of the heat retaining furnace to the feeding side of the cooling belt is Δt, from the cooling belt feeding side of the heat retaining furnace to the feeding side of the heat retaining furnace The processing time (sec) is t3, from the quench zone to the side to T0 Time (sec) is t4; S = (T11-T0) × t1 / 2

+((T11-T0)+(T12-T0))×t2/2

+((T12-T0)+(T21-T0))×△t/2

+((T21-T0)+(T22-T0))×t3/2

+(T22-T0)×t4/2

The temperature integral value S thus calculated is such that the content ratios (% by mass) of Si, Mn, P, and C in the steel are %Si, %Mn, %P, and %C, respectively, and the composition variation coefficient shown by the following formula is used. When Z, it satisfies 850+Z≦S≦1350+Z, namely:

Z=1300×(%Si-0.03)+1000×(%Mn-0.15)+35000×(%P-0.01)+1000×(%C-0.003)

Further, the Mn-P-based oxide film is coated on the surface of the alloyed hot-dip galvanized layer in an amount of Mn of 5 to 100 mg/m ² and P of 3 to 500 mg/m ² .

In the heating furnace for heating the steel sheet, the heating rate V calculated by V=(T11-T0)/t1 can be controlled as follows: when the Z is less than 700, the low-speed heating condition controlled below 100 (° C/sec) can be controlled. When the Z is 700 or more, the low-speed heating conditions of 60 (° C./sec) or less are controlled.

The steel sheet may contain C: 0.0001 to 0.3%, Si: 0.01 to 4%, Mn: 0.01 to 2%, P: 0.002 to 0.2%, and Al: 0.0001 to 4% by mass%.

According to the present invention, it is possible to obtain an alloyed hot-dip galvanized steel sheet which is excellent in uniformity of appearance, has both powdering resistance and surface slidability (flaking resistance) at the time of press molding, and is excellent in chemical conversion treatability and spot weldability.

Simple illustration

Fig. 1A is a schematic view for explaining the starting point of the occurrence of a Zn-Fe alloy (alloyed hot-dip galvanizing) in the hot-dip galvanized layer.

Fig. 1B is a schematic view showing the growth process and growth rate of a Zn-Fe alloy (alloyed hot-dip galvanizing).

Fig. 1C is a pattern diagram for explaining the texture of the alloyed hot-dip galvanized layer (difference in plating thickness).

Figure 2 is a schematic diagram showing the relationship between the alloying heating time and the thickness of the coating to illustrate the mechanism of the texture of the alloyed hot-dip galvanized layer (difference in plating thickness).

Figure 3 is a schematic diagram showing the difference in plating thickness depending on the heating rate; (a) is a pattern for explaining the difference in plating thickness during rapid heating, and (b) is used to describe the thickness of plating during low-speed heating. Poor pattern diagram.

Fig. 4 is a schematic view showing the relationship between the degree of alloying of the alloyed hot-dip galvanized layer and the enthalpy phase and enthalpy phase formed.

Fig. 5 is a schematic view showing the structure of the alloyed hot-dip galvanized steel sheet of the present invention.

Fig. 6 is a graph showing the relationship between the amount of adhesion of the film and the friction coefficient when a Mn-P-based oxide film is formed on the surface of the alloyed hot-dip galvanized steel sheet having different degrees of alloying.

Fig. 7 illustrates the process of the alloyed hot-dip galvanized steel sheet of the present invention.

Fig. 8 is a view showing an embodiment of a heating mode of the alloyed hot-dip galvanized steel sheet of the present invention.

Fig. 9 is a graph showing the relationship between the temperature integral value (S) used in the present invention and the Fe concentration in the plating layer when the composition of the steel sheet is small.

Fig. 10 illustrates the relationship between the temperature integral value (S) used in the present invention and the Fe concentration in the plating layer.

Embodiment of the present invention

The invention is described in detail below.

First, the reason for limiting each element in the steel sheet base material in the present invention will be described below. In addition, the % recited below is % by mass.

(C: 0.0001~0.3%)

The C system is required to ensure the strength, and must be contained in an amount of 0.0001% or more in order to obtain the effect. However, when it contains more than 0.3%, not only it becomes difficult to alloy, but it is difficult to ensure weldability. Therefore, the content of C must be below 0.3%. And it should be 0.001~0.2%.

(Si: 0.01~4%)

The Si-based element necessary for ensuring the ductility and strength of the steel sheet must be contained in an amount of 0.01% or more in order to obtain the effect. However, Si lowers the alloying speed and prolongs the alloying treatment time. Therefore, in order to shorten the alloying treatment time for low-speed heating, the content of Si must be 4% or less. And preferably 0.01~1%.

(Mn: 0.01~2%)

Mn is an effective element for improving the strength of the steel sheet, and must be contained in an amount of 0.01% or more in order to obtain the effect. On the other hand, if it contains more than 2%, it will adversely affect the ductility of a steel plate. Therefore, the content of Mn must be 2% or less. And should be 0.4~1.5%.

(P: 0.002~0.2%)

P is an effective element for increasing the strength of the steel sheet, and must be contained in an amount of 0.002% or more in order to obtain the effect. However, P and Si also lower the alloying speed, resulting in prolonged alloying time. Therefore, in order to shorten the alloying treatment time for low-speed heating, the content of P must be 0.2% or less.

(Al: 0.0001~4%)

From the perspective of cost, Al must contain more than 0.0001%. However, once it contains more than 4%, the alloying speed will decrease. Therefore, the content of Al must be below 4%. And it should be 0.001~2%.

Next, the cause of the appearance mottle of the alloyed hot-dip galvanized layer, that is, the mechanism of the difference in the thickness of the plating layer will be described.

The 1A~1C drawings are a pattern diagram for explaining the occurrence of the texture of the alloyed hot-dip galvanized layer (difference in plating thickness).

As shown in Fig. 1A, the alloying of the plating layer 101 is alloyed (Fe + Zn reaction) from the grain boundary 103 of the P non-concentrated portion 122 of the base iron (steel plate) 102 by alloying treatment (heating). ) 104. Since the alloying 104 is started, Fe in the steel sheet 102 and Zn in the hot-dip galvanizing 120 are mutually diffused to produce alloyed hot-dip galvanized 121. However, the unevenness of the surface of the steel sheet, that is, the P non-concentrated portion 122 and the P-concentrated portion 123 cause a difference in alloying speed. Due to this speed difference, as shown in Fig. 1B, the portion where the alloying speed is faster is lower than that of the surrounding layer, and the plating layer grows thicker (indicated by an arrow). Therefore, as shown in Fig. 1C, the thickened portion of the alloyed hot-dip galvanized steel sheet 124 is protruded to form the line of the linear spot portion 105. That is, the texture occurs due to the difference in plating thickness caused by the difference in alloying speed.

Figure 2 is a schematic diagram showing the mechanism of the texture of the alloyed hot-dip galvanized layer (difference in plating thickness).

The alloying speed (coating thickness) d is related to the diffusion coefficient D and the heating time ta, and can be expressed by the following formula (1).

The relationship between the heating time ta and the plating thickness d shown in the above formula (1) is shown in Fig. 2 . Once the alloy is heated for heating, the alloying is started in a predetermined latency set in the steel sheet composition, crystal orientation, crystal grain size, and diffusion coefficient, and the alloyed hot-dip galvanized layer is grown. However, the start time of the alloying is locally slow due to the state of the base metal, etc., and a latency difference is generated. Due to the difference in latency, a difference in plating thickness occurs and becomes a linear spot (texture).

In addition, this plating thickness difference is also affected by the heating rate.

Figure 3 is a schematic diagram showing the relationship between the thickness of the coating and the heating rate. In particular, Fig. 3(a) is used to illustrate the difference in plating thickness at the time of rapid heating, and Fig. 3(b) is a pattern for explaining the difference in plating thickness at the time of low-speed heating.

Once the alloying treatment is carried out by rapid heating, as shown in Fig. 3(a), the growth of the plating layer is accelerated. As a result, the difference in plating thickness caused by the latency difference increases. On the other hand, when the alloying treatment is performed by heating at a low speed, as shown in the third (b) diagram, the growth of the plating layer is slowed down. As a result, the difference in plating thickness caused by the latency difference is reduced. Therefore, the occurrence of texture can be suppressed, and an alloyed hot-dip galvanized layer having an optimized appearance can be formed.

As described above, it is known that the degree of alloying (coating thickness) is related to the latency and the diffusion coefficient. The greater the latency difference or the higher the heating rate, the greater the difference in plating thickness, and the linear spots (textures) become apparent.

In addition, this latency difference varies depending on the composition of the steel sheet. Therefore, when a large amount of easily segregation elements are contained in the grain boundary, and the interdiffusion speed of Fe and Zn locally changes, the difference in thickness of the plating layer occurs. Further, in relation to the addition amount of these elements, the mutual diffusion speed of Fe and Zn changes. Therefore, it is necessary to determine the heating rate V of the alloying treatment depending on the amount of addition of the elements.

Here, in the present invention, the heating of the alloying treatment is controlled to a low-speed heating condition to suppress the occurrence of linear spots (textures). Specifically, the alloying treatment is performed, and the temperature integral value (S) calculated by the formula (6), which will be described in detail later, is expressed by the formula (7), and the composition variation coefficient (Z) satisfies the formula (8), that is, 850+Z. ≦S≦1350+Z. Furthermore, it is only necessary to perform alloying treatment under the following low-speed heating conditions: when the composition variation coefficient (Z) is less than 700, the heating rate V calculated by the formula (9) is controlled to 100 ° C/sec or less. When the composition variation coefficient (Z) is 700 or more, the heating rate V is controlled to 60 ° C /sec or less. Next, the press formability will be described.

In the process of alloying a hot-dip galvanized steel sheet, first, a steel sheet annealed by an annealing furnace is immersed in a molten zinc bath, and a surface is plated to produce a hot-dip galvanized steel sheet. After the hot-dip galvanized steel sheet is heated to a maximum reaching temperature in a heating furnace, it is cooled by a heat retaining furnace, and then quenched by a cooling belt to obtain an alloyed hot-dip galvanized steel sheet. The degree of alloying is determined depending on the alloying temperature at the time of the alloying treatment.

Figure 4 shows the relationship between the degree of alloying and the enthalpy phase and enthalpy phase of the formation. As shown in Fig. 4, if the degree of alloying is low, the formation of the ζ phase is promoted and the formation of the Γ phase is suppressed. Therefore, the ζ phase is thickened and the Γ phase is thinned. On the other hand, if the degree of alloying is high, the formation of the Γ phase is promoted, and the formation of the ζ phase is suppressed. Therefore, the Γ phase is thickened and the ζ phase is thinned.

When the degree of alloying is high, the Γ phase grows, and a thick Γ phase is formed at the interface between the steel sheet and the plating layer. Therefore, the alloyed hot-dip galvanized steel sheet is pulverized during press forming. That is, once the degree of alloying is high and the Fe concentration is 10.5% or more, the Γ phase grows thick and causes pulverization. On the other hand, if the degree of alloying is low, the ζ phase of the surface of the plating layer increases, and peeling occurs during press forming. Further, if the Fe concentration is lowered, the weldability is deteriorated, which adversely affects the production process of the automobile.

The present invention is directed to suppressing the occurrence of pulverization by reducing the degree of alloying, that is, by suppressing the formation of Γ phase and promoting the formation of ζ phase. On the other hand, a method for preventing occurrence of peeling which is a problem of preventing the degree of alloying is investigated. As a result, as shown in Fig. 5, the Mn-P-based oxide film 40 was formed on the surface of the alloyed hot-dip galvanized steel sheet 24 which was low-alloyed to form an oxide film-treated alloyed hot-dip galvanized steel sheet 25 (alloyed melting) Galvanized steel sheet) can significantly improve the slidability of the surface of the steel sheet to prevent peeling. As shown in Fig. 5, the alloyed hot-dip galvanized steel sheet 25 has a steel sheet 2, an alloyed hot-dip galvanized layer 21 composed of a ζ phase 30, a δ ₁ phase 31, and a Γ phase 32, and an Mn-P system oxidation. Film 40. The alloyed hot-dip galvanized steel sheet 25 of the present invention is composed of an alloyed hot-dip galvanized steel sheet 24 and a Mn-P-based oxide film 40.

Fig. 6 is a graph showing the relationship between the amount of film adhesion and the coefficient of friction when a Mn-P-based oxide film is formed on the surface of a hot-dip galvanized steel sheet having different degrees of alloying.

The IF steel cold-rolled steel sheet or the high-strength steel cold-rolled steel sheet is subjected to hot-dip galvanizing, alloyed under different alloying conditions, and the heating rate is changed. By this treatment, a hot-dip galvanized steel sheet having a low degree of alloying and a hot-dip galvanized steel sheet having a high degree of alloying are prepared. The Mn-P-based oxide film was adhered to the steel sheets as a lubricating film, and the individual friction coefficients were investigated.

The compressive friction coefficient is based on sample size = 17 mm × 300 mm, tensile speed: 500 mm/min, square bead shoulder R: 1.0 / 3.0 mm, sliding length: 200 mm, oil coating: NOX-RUST 530F-40 (Parks Hyundai Co., Ltd., Parkers Industries, Inc.) Under the conditions of oil application amount of 1 g/m ² , the surface pressure was tested between 100 and 600 kgf, and the drawing weight was measured. The coefficient of friction is obtained from the gradient of the surface pressure and the withdrawal weight.

As shown in Fig. 6, the hot-dip galvanized steel sheet (δ ₁ + ζ phase main body) having a low degree of alloying has a higher friction coefficient and a lower surface slidability than the hot-dip galvanized steel sheet having a higher degree of alloying. However, when a Mn-P-based oxide film is formed on the surface, the amount of adhesion is small and the friction coefficient is remarkably lowered as compared with the hot-galvanized steel sheet having a high alloying degree. Thus, by lowering the degree of alloying and increasing the ζ phase, the slidability can be improved by the smaller amount of Mn-P-based oxide film adhesion. Further, even if a predetermined amount of the film is applied, the molten galvanized steel sheet having a low alloying degree can maintain excellent slidability as compared with the hot-galvanized steel sheet having a high alloying degree. This is considered to be because the coating layer of the hot-dip galvanized steel sheet having a low degree of alloying has a small Fe concentration. However, the detailed mechanism is still unclear.

In the present invention, by reducing the degree of alloying, suppressing the formation of the Γ phase and promoting the formation of the ζ phase, the occurrence of pulverization can be suppressed. Further, by imparting an inorganic lubricating film to the Mn-P-based oxide film, it is possible to suppress the occurrence of peeling which has been an industrial problem.

The alloying degree of alloying hot-dip galvanizing is determined by the alloying temperature, heating time, and cooling conditions. The alloyed hot-dip galvanized steel sheet having a low degree of alloying with a large amount of bismuth phase can be generally obtained by the following heat treatment conditions. That is, after the steel sheet is subjected to hot-dip galvanizing, it is heated by an induction heating device at a heating rate of 40 to 70 ° C/sec from 500 to 670 ° C. The alloyed hot-dip galvanized steel sheet is maintained at an alloying temperature of 440 to 530 ° C for 5 to 20 seconds, and the Fe content in the Zn-Fe alloy is adjusted to 6.5 to 13%, and preferably 9.0 to 10.5%.

When the Fe content is less than 9.0%, the degree of alloying is insufficient, and the weldability is lowered, which is not preferable. On the other hand, once the Fe content exceeds 10.5%, the powdering resistance is deteriorated due to an increase in the Γ phase.

The diffraction intensity of the Zn phase, δ ₁ phase, and ζ phase of the Zn-Fe alloy phase of the alloyed hot-dip galvanized steel sheet of such low alloying degree in X-ray diffraction was investigated, and it was found that the target of the present invention was It is important that the alloyed hot-dip galvanized layer is such that the diffraction intensity of the Γ phase, the diffraction intensity of the δ ₁ phase, and the diffraction intensity of the ζ phase satisfy the phase structures of the following formulas (2) and (3), respectively.

Γ (2.59 ) / δ ₁ (2.13 )≦0.1‧‧‧‧‧(2)

0.1≦ζ (1.26 ) / δ ₁ (2.13 )≦0.4‧‧‧(3)

That is, in the above formula, Γ (2.59 ) / δ ₁ (2.13 ) must be below 0.1. When the value exceeds 0.1, the hard and brittle Γ phase in the interface between the plating layer and the steel sheet increases, and the powdering resistance of the alloyed hot-dip galvanized steel sheet at the time of press forming deteriorates. In addition, ζ (1.26 ) / δ ₁ (2.13 ) must be 0.1 or more and 0.4 or less. When the value is less than 0.1, the ζ phase is reduced, and when the Mn-P-based oxide film is applied, the slidability improving effect beyond the conventional material cannot be exhibited. On the other hand, once ζ (1.26 ) / δ ₁ (2.13 When it exceeds 0.4, the amount of unalloyed Zn will increase, and the weldability will decrease.

More preferably, the alloyed hot-dip galvanized layer as the target in the present invention has a phase structure in which the diffraction intensity of the Γ phase and the diffraction intensity of the ζ phase satisfy the following formulas (4) and (5), respectively.

Γ (2.59 )≦100(cps)‧‧‧‧‧(4)

100≦ζ (1.26 )≦300(cps)‧‧‧(5)

The phase structure of the alloyed hot-dip galvanized layer can be determined by measuring the diffraction intensity of the Γ phase, the δ ₁ phase, and the ζ phase by X-ray diffraction. Specifically, the plating layer and the iron plate are bonded together using an epoxy-based adhesive to cure the adhesive, and then mechanically pulled to peel the plating layer from the interface of the base iron together with the adhesive. For the peeled plating layer, X-ray diffraction was performed from the interface side between the plating layer and the steel sheet, and the diffraction peak of the alloy phase was measured.

The conditions for X-ray diffraction are measurement surfaces: a perfect circular shape with a diameter of 15 mm, a θ/2θ method, an X-ray tube ball: a Cu tube ball, a tube voltage: 50 kV, and a tube current: 250 mA. Under these conditions, the diffraction peak of the alloy phase was determined to be derived from the interfacial phase (Fe ₃ Zn ₁₀ ) and the Γ ₁ phase (Fe ₅ Zn ₂₁ ). The lattice spacing d = 2.59 Diffraction intensity (cps): Γ (2.59 ), considered to be derived from the δ ₁ phase (FeZn ₇ ) lattice spacing d = 2.13 Diffraction intensity (cps): δ ₁ (2.13 ), and the lattice plane spacing considered to be derived from the ζ phase (FeZn ₁₃ ) is d=1.26 Diffraction intensity (cps): ζ (1.26 ). Further, since it is difficult to distinguish the Γ phase from the Γ ₁ phase by crystallography, in the present invention, the Γ phase and Γ _{1 are} collectively regarded as the Γ phase.

As a method for producing a alloyed hot-dip galvanized steel sheet having a particularly good low alloying degree according to the present invention, the temperature (T) during heating and cooling in the alloying treatment can be multiplied by time (t), and calculated according to the multiplication. The temperature integral value (S) is determined after the temperature mode in the alloying treatment described above is performed.

That is, the hot-dip galvanized steel sheet is heated in a heating furnace, and after reaching the steel sheet temperature (T11) of the highest reaching temperature on the feeding side of the heating furnace, it is cooled by the heat retaining furnace. In the condition of the alloying treatment described above, the temperature integral value (S) calculated by the formula (6) can be satisfied by the following formula (8) using the composition variation coefficient (Z) calculated by the following formula (7). , ie 850+Z≦S≦1350+Z.

According to this manufacturing method, the alloyed hot-dip galvanized steel sheet having a low alloying degree having a phase structure of a predetermined Fe content can be easily obtained.

S=(T11-T0)×t1/2+((T11-T0)+(T12-T0))×t2/2+((T12-T0)+(T21-T0))×Δt/2+((T21-T0) )+(T22-T0))×t3/2+(T22-T0)×t4/2 ‧‧‧‧‧‧‧‧‧‧ (6)

Here, the meaning of each symbol in the formula is as follows.

T0: 420 (°C)

T11: Steel plate temperature on the delivery side of the furnace (°C)

T12: Steel plate temperature (°C) on the entry side of the cooling zone of the heat preservation furnace

T21: Steel plate temperature on the delivery side of the cooling belt (°C)

T22: Steel plate temperature on the delivery side of the furnace (°C)

T1: processing time from T0 to the sending side of the furnace (sec)

T2: processing time from the feeding side of the heating furnace to the inlet side of the cooling zone of the heat retaining furnace (sec)

Δt: processing time from the entry side of the cooling zone of the heat retaining furnace to the delivery side of the cooling zone (sec)

T3: processing time from the cooling belt delivery side of the thermal furnace to the delivery side of the thermal furnace (sec)

T4: processing time from the quench zone entry side to T0 (sec)

Z=1300×(%Si-0.03)+1000×(%Mn-0.15)+35000×(%P-0.01)+1000×(%C-0.003)‧‧‧(7)

Here, %Si, %Mn, %P, and %C respectively indicate the content ratio (% by mass) of Si, Mn, P, and C in the steel.

850+Z≦S≦1350+Z‧‧‧(8)

The reason why the temperature integral value (S) satisfies the condition of the formula (8) is as follows. When the temperature integral value (S) is less than 850+Z, ζ (1.26) ) / δ ₁ (2.13 ) will be greater than 0.4 to deteriorate the weldability. On the other hand, once the temperature integral value (S) exceeds 1350+Z, because Γ (2.59 ) / δ ₁ (2.13 ) will be greater than 0.1 and the pulverizability will deteriorate.

Further, regarding the heating rate, the heating rate up to the steel sheet temperature (T11) on the feeding side of the heating furnace, that is, the heating speed V (° C/sec) shown by the following formula (9), greatly affects the appearance. Therefore, when the composition variation coefficient (Z) is less than 700, the heating rate V calculated by the formula (9) is 100 ° C /sec or less. Further, when the composition variation coefficient (Z) is 700 or more, the heating rate V is made 60 ° C/sec or less. A plated steel sheet having a good appearance can be obtained by controlling this heating rate V. The lower limit of V is not particularly limited, but generally, it is set to 30 ° C /sec or more in order to maintain S at a predetermined value.

V=(T11-T0)/t1‧‧‧‧‧(9)

Here, T0: 420 (° C.), T11: steel sheet temperature (° C.) on the feeding side of the heating furnace, and t1 : processing time (sec) after the steel sheet temperature reaches T0 to the heating furnace feeding side.

Fig. 7 illustrates the process of the alloyed hot-dip galvanized steel sheet in the present invention.

First, the steel sheet 2 which has been annealed by the annealing furnace 6 is immersed in a molten zinc pot 8 to apply a plating layer on the surface. Further, the hot-dip galvanized steel sheet 2A is heated to the highest temperature reached by the heating furnace 9, and then cooled by the heat retaining furnace 10, and then quenched by the quenching belt 11 to obtain an alloyed hot-dip galvanized steel sheet 24. At this time, there is also a case where the heat retention furnace 10 is forcibly cooled for a fixed period of time. The right diagram of Fig. 7 illustrates the heating mode of the alloyed hot-dip galvanized steel sheet in the process. First, once the steel sheet 2 is immersed in a plating bath, an Fe-Al alloy phase (Al barrier layer) is initially formed, which will become a barrier to the alloying reaction between Fe and Zn. The hot-dip galvanized steel sheet 2A leaving the plating bath is cooled in the process of controlling the amount of plating adhesion, and then heated to the highest reaching temperature in a heating furnace. This heating process determines the initial phase of the Fe-Zn alloy. Next, the diffusion of Fe and Zn occurs during the process of cooling the furnace, and the plating structure is determined.

Fig. 8 is a view showing an embodiment of a heating mode of the alloyed hot-dip galvanized steel sheet according to the present invention.

First, a plated steel sheet (temperature T0) to which a plating layer is applied by immersing in a zinc plating bath at a steel sheet temperature (Tin) is heated in a heating furnace to a steel sheet temperature (T11). Thereafter, the plated steel sheet is cooled in a heat-retaining furnace divided into two. First, after leaving the heating furnace, the plated steel sheet is charged into the first heat retaining furnace at a temperature of T12, and cooled by a cooling device (cooling belt) from T12 to a temperature of T21. This cooling can also be omitted.

Next, the plated steel sheet is cooled to a temperature of T22 in the second heat retaining furnace, and then cooled to a temperature T0 by a quench zone.

The inventor of the present invention analyzed the relationship between the temperature integral value (S) and the plating structure in the present invention, and found that the temperature integral value (S) satisfies the equations (7) and (8), that is, 850+Z≦S≦1350+. Z and Z = 1300 × (% Si - 0.03) + 1000 × (% Mn - 0.15) + 35000 × (% P - 0.01) + 1000 × (% C - 0.003), and if the composition variation coefficient (Z) is less than 700 Then, the heating rate V calculated by the formula (9) is 100° C./sec or less, and when the composition variation coefficient (Z) is 700 or more, the heating rate V is set to 60° C./sec or less, and the heating mode is adjusted as described above. and make accessible having excellent coating appearance ζ-phase structure containing the desired product characteristics.

In the present embodiment, the temperature integral value (S) is obtained from the Fe concentration, and The pass speed (LS) determines the above t1~t4, and is determined by the condition of the heat retaining furnace (T11-T22), and then T11 and T22 are determined according to the values and Δt.

Further, when the heat retaining furnace is not provided with a cooling belt, the Δt in the above formula (6) may be 0.

Next, the concept of the temperature integral value (S) in the present invention will be described below.

First, the diffusion coefficient D and the diffusion distance X of the alloy plating layer are expressed by the following formula (10) and the following formula (11), respectively.

D=D0×exp(-Q/R‧T)‧‧‧(10)

Here, D: diffusion coefficient, D0: constant, Q: activation energy of diffusion, R: gas constant, T: temperature, X: diffusion distance (infiltration depth), t: time.

The above equation (10) is approximated by Taylor expansion and is D. (A+B‧T). Substituting this into the formula (11), the following formula (12) is obtained.

It can be known from equation (12) that the diffusion distance (X) can represent the Fe concentration in the alloy coating, so the temperature (T) is multiplied by the time (t), and the obtained temperature integral value (S) is multiplied with the alloy plating layer. Fe concentration is related.

The order of determination of the alloying conditions in the present invention is exemplified as follows.

The method for determining the alloying conditions is as follows. First, a relational expression between the temperature integral value (S) and the Fe concentration in the plating layer is obtained. From the equation and the theoretical formula for calculating the temperature integral value (S), the correlation between the degree of alloying and the steel sheet temperature (T11) on the feed side of the furnace, T11 = f (alloying degree (Fe concentration), steel type, and adhesion amount) are derived. , steel plate speed, plate thickness). In addition, according to the changes of various parameters, the steel plate temperature (T11) of the optimum heating furnace feeding side is automatically calculated at any time. The heat input to the furnace is adjusted to maintain the calculated steel sheet temperature on the exit side of the optimum furnace.

<take data>

(i) Determine the minimum value of the temperature integral value (S) that can be alloyed under each condition (steel type, adhesion amount, steel sheet speed, thickness), and derive the influence coefficient of the steel grade on the steel sheet temperature on the optimum heating furnace delivery side.

(ii) The temperature of the steel sheet on the feeding side of the heating furnace is changed, and the correlation between the temperature integral value (S) and the Fe concentration (alloying degree) in the plating layer is obtained, and S = f (Fe% in the plating layer) is derived.

The ninth system exemplifies the temperature integral value used in the present invention when the Si, Mn, P, and C contents (% by mass) in the IF steel material are % Si = 0.01, % Mn = 0.01, % P = 0.005, and % C = 0.001, respectively. S) is related to the concentration of Fe in the coating.

In addition, FIG. 10 illustrates that the Si, Mn, P, and C contents (% by mass) in the high-strength steel material are used in the present invention when %Si=0.03, %Mn=0.15, %P=0.02, and %C=0.003, respectively. The relationship between the temperature integral value (S) and the Fe concentration in the coating.

As shown in Fig. 9 and Fig. 10, the relationship between the temperature integral value (S) and the Fe concentration in the plating layer varies depending on the composition of the steel. When the composition condition of the steel changes, the coefficient for correcting the relationship between the temperature integral value (S) and the Fe concentration in the plating layer is the composition variation coefficient (Z). Therefore, when the composition of the steel changes, the S value may be corrected by adding the composition variation coefficient (Z) calculated by the equation (7) to the S value.

As described above, in the figures 9 and 10, the temperature integral value (S) of the IF steel material or the high-strength steel material having a plating weight (plating adhesion amount) of 40 to 50 mg/m ² is correlated with the Fe concentration in the plating layer. Therefore, the equation (a) can be derived by obtaining an approximate expression from the correlation.

Fe%=f(S) ‧‧‧‧(a)

By using the formula (a), the temperature integral value (S) can be determined by the following formula (b) in accordance with the target Fe concentration in the alloy plating layer.

S=f (Fe concentration) ‧‧‧‧(b)

(iii) Using the empirical data, the prediction formula of the steel sheet temperature (T22) on the delivery side of the heat retaining furnace is derived.

According to the empirical data of the ninth and tenth graphs, the difference between the steel sheet temperature (T11) on the heating furnace feeding side and the steel sheet temperature (T22) on the heating furnace feeding side obtained by the multiple regression calculation is the formula (c).

T11-T22=f (passing plate speed, plate thickness) ‧‧‧(c)

In the case of cooling in the heat preservation furnace, it is usually cooled to about 5 to 30 ° C, but the temperature drop amount T12-T21 of this part can also be contained in T11-T22 to determine the temperature mode.

<Data Analysis>

(iv) In the above formula (6) of the definition formula of the temperature integral value (S), the empirical value of the figures 9 and 10 is substituted, and in the following formula (d), the above formula (b) is substituted. And formula (c). Thereby, S=f (steel plate temperature, plate speed, plate thickness on the heating furnace feeding side), formula (d) and formula (e) can be derived.

S=f (passing speed, T11, T22) ‧‧‧(d)

T11=f (passing plate speed, plate thickness, Fe concentration) ‧‧‧(e)

(v) The plating weight (coating adhesion amount) and the Fe concentration will be established once. Therefore, the influence of the amount of adhesion on the temperature of the steel sheet on the delivery side of the heating furnace is determined, and the Fe concentration of the formula (b) is replaced by the Fe concentration + α ‧ Δ coating weight to obtain the formula (f).

T11=f (passing plate speed, plate thickness, Fe concentration, adhesion amount) ‧‧‧(f)

Here, α represents the gradient of the above correlation formula, and the Δ plating weight represents the plating weight increase amount with respect to the plating weight reference value.

(vi) The influence coefficient of the steel grade obtained in (i) on the steel sheet temperature on the feeding side of the optimum heating furnace is added to the formula (f), and the formula (g) can be obtained. At this time, the value of T11 is set such that the aforementioned V value does not exceed a predetermined value (60 ° C/sec or 100 ° C/sec) determined by the composition variation coefficient (Z).

T11=f (passing plate speed, sheet thickness, Fe concentration, adhesion amount, steel type) ‧‧‧(g)

By the formula (g), the steel sheet temperature (T11) on the feed side of the heating furnace is determined in accordance with the temperature integral value (S) determined as described above. Therefore, even if the steel sheet thickness and/or the sheet speed, the plating weight, the alloying degree (Fe concentration), and the steel type change, the heat input to the heating furnace can be adjusted to maintain the steel sheet temperature (T11) on the heating furnace feeding side.

The control flow when the present invention is implemented will be described below.

First, the steel grade, the steel plate size, the upper and lower limits of the adhesion amount, and the alloying degree are transmitted from the first computer to the second computer. Next, by the second computer, the side panel temperature control formula is sent by IH to calculate influence items other than the board speed (LS), and transmitted to the control device.

The control device will calculate the above-mentioned board speed (LS) influence term to calculate the IH send side panel temperature and determine the IH output power. The control device further transmits the IH input and output panel temperature setting value, experience value, power experience value, and the like to the computer 2.

Next, the second computer calculates the alloying quality by the difference between the IH sent the side panel temperature empirical value (T11) and the second computer calculated IH delivery side panel temperature setting value. Further, the second computer transmits the 1H input/outlet panel temperature setting value, the experience value, the power experience value, and the like to the first computer. The first computer automatically retains the coil of the quality judgment NG made by the second computer. In addition, the first computer saves the experience values in the database.

As described above, after the galvanized steel sheet is heated to the temperature of the steel sheet on the side of the furnace at the highest temperature (T11), the furnace is cooled by the heat retaining furnace, and the temperature integral value (S) calculated by the formula (6) is used. 7) The calculated composition variation coefficient (Z) satisfies the formula (8), that is, the alloying treatment under the condition of 850+Z≦S≦1350+Z, whereby the low alloyed hot-dip galvanized steel sheet of the present invention can be effectively obtained. .

Next, the Mn-P-based oxide film formed on the alloyed hot-dip galvanized steel sheet having a low alloying degree will be described.

In the present invention, in order to improve the surface slidability of the alloyed hot-dip galvanized steel sheet having a low alloying degree and prevent peeling during press forming, the Mn-P-based oxide film is formed as a lubricious hard film. On the surface of the steel plate. It was found that a small amount of the oxide film can be attached as shown in Fig. 6, whereby the surface slidability is remarkably improved. In order to improve the adhesion of the oxide film and the film formability, a P-containing aqueous solution is mixed. By this film formation method, a Mn-P-based oxide film is produced and the structure is made uniform, so that film formability and lubricity are improved. Therefore, the press formability is further improved, and the chemical conversion treatability is also improved. Further, the Mn-P-based oxide film is the same as the chromate film, and both of them become a glassy film. When pressed, the adhesion of the plating layer to the mold is suppressed, and the slidability is improved. Further, since the Mn-P-based oxide film is dissolved in the chemical conversion treatment liquid, unlike the chromate film, the chemical conversion treatment film can be easily formed. Further, since the Mn-P-based oxide film is also a component of the chemical conversion treatment film, it has no adverse effect even if it is eluted into the chemical conversion treatment liquid, and the chemical conversion treatment property is good.

Although the structure of the Mn-P-based oxide film is not clear, it is conceivable that a network composed of Mn-O bonding and PO bonding is mainly used. Further, it is estimated that a part of the inside of the network contains an OH, a CO ₂ group or the like to form an amorphous giant molecular structure which has been replaced with a metal supplied from the plating layer.

Next, as a method of forming the oxide film, for example, a method of immersing a steel sheet in an aqueous solution containing a Mn aqueous solution, a P-containing aqueous solution, an etching aid (sulfuric acid or the like), a method of dispersing an aqueous solution, and a steel sheet The desired oxide film can be produced by a method in which the cathode is electrolytically treated in an aqueous solution.

In order to obtain good press formability, the amount of the film of the Mn-P-based oxide may be 5 mg/m ² or more in terms of Mn. However, if the amount of the film exceeds 100 mg/m ² , the formation of the chemical conversion treatment film is uneven. Therefore, the amount of the film to be used is 5 mg/m ² or more and 100 mg/m ² or less in terms of Mn. In particular, in the alloyed hot-dip galvanized steel sheet having a low alloying degree, the smaller the amount of adhesion, the better the slidability. Although the reason is not clear, the alloyed hot-dip galvanized layer having a small Fe content and the layer directly reacting with Mn are most effective in improving the slidability. Therefore, a preferred amount of Mn adhesion is 5 to 70 mg/m ² . In addition, the P deposition amount is 3 mg/m ² or more in terms of P, and the film formation property of the Mn oxide is improved, and the effect of improving the slidability is further exhibited. However, when the P adhesion amount exceeds 500 mg/m ² , it is not preferable because the chemical conversion treatability is deteriorated. Therefore, the preferred P adhesion amount is 3 to 200 mg/m ² .

A Mn-P-based oxide film is formed on a low-alloyed alloyed hot-dip galvanized steel sheet as a hard coating for lubrication, thereby achieving both powdering resistance, surface slidability (flaking resistance), and chemical conversion. Alloyed hot-dip galvanized steel sheet excellent in handleability and spot weldability.

Example

Next, the invention will be described in more detail by way of examples.

(melt plating)

The steel which has changed the C, Si, Mn, P, and Al in the steel is placed in a 10% H ₂ -N ₂ gas atmosphere and subjected to reduction and annealing treatment at 800 ° C for 90 seconds. Further, it was immersed in a Zn plating bath containing 460 ° C of Al = 0.13% and Fe = 0.025% for 3 seconds to carry out plating. Thereafter, the amount of plating adhesion was controlled by a gas wiping method to a basis weight of 45 g/m ² . The plated steel sheet is heated to the steel sheet temperature (T11) on the heating furnace delivery side of the highest temperature, and then cold-cooled in a heat retaining furnace to perform alloying treatment. In the alloying treatment, the temperature integral value (S) calculated by the formula (6) is variously changed to produce a alloyed hot-dip galvanized steel sheet having various alloying degrees.

(Exterior)

By visual inspection, the appearance uniformity is evaluated as good, the partial unevenness is evaluated as the fair, and the overall unevenness is evaluated as not good.

(Oxide film treatment)

The following treatment was carried out in order to produce an oxide film. As the electrolytic bath, a mixed solution containing a Mn aqueous solution, a P-containing aqueous solution, sulfuric acid and zinc carbonate at 30 ° C was used, a treated steel sheet was used for the cathode, and a Pt electrode was used for the anode, and electrolysis was performed at 7 A/dm ² for 1.5 seconds. Thereafter, the steel sheet to be treated is washed with water and dried, and the concentration of the Mn-containing aqueous solution, the P-containing aqueous solution, the sulfuric acid and the zinc carbonate, the solution temperature, and the immersion time are adjusted, and immersed in the mixed solution to produce an oxide film.

(plating structure)

Measuring surface: a perfect circular shape with a diameter of 15 mm

θ/2θ method

X-ray tube ball: Cu tube ball

Tube voltage: 50kV

Tube current: 250mA

In the diffraction peak of the alloy phase, it is determined that the lattice spacing from the Γ phase (Fe ₃ Zn ₁₀ ) and the Γ ₁ phase (Fe ₅ Zn ₂₁ ) is d=2.59 Diffraction intensity (cps): Γ (2.59 ), considered to be derived from the δ ₁ phase (FeZn ₇ ) lattice spacing d = 2.13 Diffraction intensity (cps): δ ₁ (2.13 And the lattice plane spacing considered to be derived from the ζ phase (FeZn ₁₃ ) is d=1.26 Diffraction intensity (cps): ζ phase (1.26 ).

Further, since it is difficult to distinguish the Γ phase from the Γ ₁ phase in crystallography, in the present invention, the Γ phase and the Γ ₁ phase are combined and recorded as the Γ phase.

Γ (2.59 ): lattice spacing d = 2.59 Diffraction intensity

δ ₁ (2.13 ): lattice spacing d = 2.13 δ ₁ phase diffraction intensity

ζ (1.26 ): lattice spacing d = 1.26 Diffraction intensity

(pulverized)

A crank press was used, and a alloyed hot-dip galvanized steel sheet (GA) having a width of 40 mm and a length of 250 mm was used as a test material, and a semicircular bead film with r = 5 mm was used, and a punching shoulder radius of 5 mm and a die shoulder radius of 5 mm were processed to The forming height is 65 mm. At the time of processing, the peeled plating was measured and evaluated on the basis of the following criteria.

Evaluation basis

Coating peeling amount: less than 5g/m ² : excellent (very good)

5g/m ² or more and less than 10g/m ² : good

10g/m ² or more and less than 15g/m ² :fair

15g/m ² or more: not good

(slidability)

Sample size = 17mm × 300mm, tensile speed: 500mm / min, square beaded shoulder R: 1.0 / 3.0mm, sliding length: 200mm, oiling: NOX-RUST 530F-40 (Parkes Industrial Co., Ltd.) Under the condition of oil application amount of 1 g/m ² , the test was carried out at a surface pressure of 100 to 600 kgf, and the drawing was measured to increase the weight. The friction coefficient is obtained from the gradient of the surface pressure and the drawing weight. The coefficient of friction obtained was evaluated on the basis of the following criteria.

Evaluation basis

Less than 0.5: excellent (very good)

0.5 or more, less than 0.6: good (good)

0.6 or more, less than 0.8: fair (fair)

0.8 or more: not good

(Chemical conversion treatment)

5D5000 (manufactured by NIPPON PAINT Co., Ltd.) was used for the chemical conversion treatment liquid (zinc-phosphoric acid-fluorine treatment bath), and the plated steel sheet was degreased and surface-adjusted according to the prescription, and then subjected to chemical conversion treatment. The chemical conversion treatability was determined as follows: the chemical conversion film was observed by SEM (secondary electron image), and the film was uniformly formed to be good (good), and the film was partially determined to be fair, and the film was not formed. Not good.

(spot weldability)

Under pressure: 2.01kN, energization time: Ts=25cyc., Tup=3cyc., Tw=8cyc., Th=5cyc., To=50cyc., wafer: DR6 spherical, etc. Direct spot welding, while making current When the value changes, the resulting nugget diameter is measured. Produced relative to the thickness td The current of the above nugget is the lower limit current, and the current of the dust generation is the upper limit current, and the difference between the upper limit current and the lower limit current, that is, the appropriate current is obtained. After confirming that the adaptive current range is 1 kA or more, the fixed current value of 0.9 times the upper limit current value is continuously welded under the above-described welding conditions. The generated nugget diameter is measured, and the nugget diameter is determined. The following counts. Those who scored more than 1000 points are good, and those less than 1000 are bad.

The test results obtained above are collectively shown in Tables 1 and 2. Table 1 is a condition in which C, Si, Mn, and P in steel are fixed in the condition of Figure 9 (ie, representative composition conditions of IF steel), and the temperature integral value (S), Mn adhesion amount, and P adhesion amount are controlled. Form. Since the steel sheet of Table 1 is a mild steel having a small amount of alloy component added, % Si = 0.01, % Mn = 0.01, % P = 0.005, % C = 0.001, and Z value is -300. Therefore, the appearance was uniform regardless of either of the examples and the comparative examples. As shown in Table 1, any of the embodiments of the present invention has excellent powdering resistance and resistance. Exfoliation (slidability), and alloyed hot-dip galvanized steel sheet excellent in chemical conversion treatability and spot weldability. On the other hand, the comparative example which did not satisfy one of the requirements of the present invention was inferior in one of powdering resistance, peeling resistance, chemical conversion treatability, and spot weldability.

Table 2 is a table for controlling the temperature integral value (S), the Mn adhesion amount, and the P adhesion amount by using a steel material in which steel, C, Si, Mn, and P have been changed. As shown in Table 2, any of the examples of the present invention has an excellent appearance, excellent powdering resistance, peeling resistance (slidability), and alloying melting with excellent chemical conversion treatability and spot weldability. Galvanized steel. On the other hand, the comparative example which did not satisfy one of the requirements of the present invention was inferior in one of appearance, powdering resistance, peeling resistance, chemical conversion treatability, and spot weldability.

Industrial availability

The present invention can provide an alloyed hot-dip galvanized steel sheet which is excellent in peeling resistance and powdering resistance, excellent in surface appearance, and excellent in chemical conversion treatability, and a method for producing the same.

2. . . Steel plate

8. . . Molten zinc bath

9. . . Heating furnace

10. . . Heat preservation furnace

11. . . Quenching zone

twenty one. . . Alloyed hot-dip galvanized layer (Zn-Fe alloy)

twenty four. . . Alloyed hot-dip galvanized steel sheet

25. . . Alloyed hot-dip galvanized steel sheet (alloyed hot-dip galvanized steel sheet) treated by oxide film

30. . . Prime minister

31. . . δ ₁ phase

32. . . Prime minister

40. . . Mn-P oxide film

Fig. 1A is a schematic view for explaining the starting point of the occurrence of a Zn-Fe alloy (alloyed hot-dip galvanizing) in the hot-dip galvanized layer.

Fig. 1B is a schematic view showing the growth process and growth rate of a Zn-Fe alloy (alloyed hot-dip galvanizing).

Fig. 1C is a pattern diagram for explaining the texture of the alloyed hot-dip galvanized layer (difference in plating thickness).

Figure 2 is a schematic diagram showing the relationship between the alloying heating time and the thickness of the coating to illustrate the mechanism of the texture of the alloyed hot-dip galvanized layer (difference in plating thickness).

Figure 3 is a schematic diagram showing the difference in plating thickness depending on the heating rate; (a) is a pattern for explaining the difference in plating thickness during rapid heating, and (b) is used to describe the thickness of plating during low-speed heating. Poor pattern diagram.

Fig. 4 is a schematic view showing the relationship between the degree of alloying of the alloyed hot-dip galvanized layer and the enthalpy phase and enthalpy phase formed.

Fig. 5 is a schematic view showing the structure of the alloyed hot-dip galvanized steel sheet of the present invention.

Fig. 6 is a graph showing the relationship between the amount of adhesion of the film and the friction coefficient when a Mn-P-based oxide film is formed on the surface of the alloyed hot-dip galvanized steel sheet having different degrees of alloying.

Fig. 7 illustrates the process of the alloyed hot-dip galvanized steel sheet of the present invention.

Fig. 8 is a view showing an embodiment of a heating mode of the alloyed hot-dip galvanized steel sheet of the present invention.

Fig. 9 is a graph showing the relationship between the temperature integral value (S) used in the present invention and the Fe concentration in the plating layer when the composition of the steel sheet is small.

Fig. 10 illustrates the relationship between the temperature integral value (S) used in the present invention and the Fe concentration in the plating layer.

2. . . Steel plate

twenty one. . . Alloyed hot-dip galvanized layer (Zn-Fe alloy)

twenty four. . . Alloyed hot-dip galvanized steel sheet

25. . . Alloyed hot-dip galvanized steel sheet (alloyed hot-dip galvanized steel sheet) treated by oxide film

30. . . Prime minister

31. . . δ ₁ phase

32. . . Prime minister

40. . . Mn-P oxide film

Claims (7)

An alloyed hot-dip galvanized steel sheet, comprising: a steel plate, an alloyed hot-dip galvanized layer and a Mn-P-based oxide film, wherein the steel plate has C, Si, Mn, P, Al, residual Fe and The component composition of the impurity to be avoided; the Zn-Fe alloy phase in the alloyed hot-dip galvanized layer is divided by the diffraction intensity Γ(2.59Å) of the Γ phase in the X-ray diffraction d=2.59Å The diffraction intensity δ ₁ (2.13 Å) of the δ ₁ phase with a lattice spacing d = 2.13 Å is 0.1 or less; the lattice spacing d = 1.26 Å is the diffraction intensity of the ζ phase (1.26 Å) divided by the crystal The δ ₁ phase diffraction intensity δ ₁ (2.13 Å) of the lattice spacing d=2.13 Å is 0.1 or more and 0.4 or less; and the Mn-P oxide film is Mn of 5 to 100 mg/m ² , P is applied to the surface of the alloyed hot-dip galvanized layer in an amount of 3 to 500 mg/m ² . The alloyed hot-dip galvanized steel sheet according to the first aspect of the patent application, wherein the steel sheet contains, by mass%, C: 0.0001 to 0.3%, Si: 0.01 to 4%, Mn: 0.01 to 2%, and P: 0.002 to 0.2. %, and Al: 0.0001~4%. The alloyed hot-dip galvanized steel sheet according to claim 1, wherein the Zn-Fe alloy phase in the alloyed hot-dip galvanized layer is surrounded by X-rays The aforementioned diffraction intensity Γ(2.59Å) of the aforementioned Γ phase of the lattice interval d=2.59Å is 100 (cps) or less, and the aforementioned diffraction intensity of the aforementioned ζ phase of the lattice plane spacing d=1.26Åζ (1.26 Å) is 100 (cps) or more and 300 (cps) or less. The alloyed hot-dip galvanized steel sheet according to the first aspect of the invention, wherein the Fe content in the Zn-Fe alloy phase in the alloyed hot-dip galvanized layer is 9.0 to 10.5%. The invention relates to a method for manufacturing an alloyed hot-dip galvanized steel sheet, which is characterized in that: the steel plate is melt-galvanized and heated in a heating furnace, and after the temperature of the steel plate on the sending side of the heating furnace reaches the highest reaching temperature, the heat-preserving furnace is used to perform the heating. Cold alloying treatment to form an alloyed hot-dip galvanized layer, and a Mn-P-based oxide film containing Mn and P is formed on the surface of the alloyed hot-dip galvanized layer; the alloying treatment is 420 (° C.) T0, the steel plate temperature (°C) on the feeding side of the heating furnace is T11, the steel plate temperature (°C) on the inlet side of the cooling zone of the heat retaining furnace is T12, and the steel plate temperature (°C) on the sending side of the cooling zone is T21, the aforementioned heat retaining furnace The steel sheet temperature (°C) on the delivery side is T22, and the treatment time (sec) from the T0 to the heating furnace delivery side is t1, and the processing time from the heating furnace delivery side to the inlet side of the cooling belt of the heat retention furnace (sec) And t2, the processing time (sec) from the inlet side of the cooling belt of the heat retaining furnace to the feeding side of the cooling belt is Δt, from the cooling belt feeding side of the heat retaining furnace to the feeding side of the heat retaining furnace Processing time (sec) is t3, entering from the quench zone The processing time (sec) of T0 is t4; S=(T11-T0)×t1/2 +((T11-T0)+(T12-T0))×t2/2+((T12-T0)+(T21- T0)) × Δt / 2 + ((T21 - T0) + (T22 - T0)) × t3 / 2 + (T22 - T0) × t4 / 2 to calculate the temperature integral value S in the steel in the steel When the content ratios (% by mass) of Mn, P, and C are %Si, %Mn, %P, and %C, respectively, and the composition variation coefficient Z shown by the following formula is used, 850+Z≦S≦1350+Z is satisfied. That is, Z = 1300 × (% Si - 0.03) + 1000 × (% Mn - 0.15) + 35000 × (% P - 0.01) + 1000 × (% C - 0.003) and the Mn-P-based oxide film is An amount of Mn of 5 to 100 mg/m ² and P of 3 to 500 mg/m ² is coated on the surface of the alloyed hot-dip galvanized layer. A method for producing an alloyed hot-dip galvanized steel sheet according to claim 5, wherein the heating rate V calculated by V = (T11 - T0) / t1 is controlled as follows in the heating furnace for heating the steel sheet: When Z is less than 700, it is controlled to a low-speed heating condition of 100 (C/sec) or less; and when Z is 700 or more, a low-speed heating condition of 60 (C/sec) or less is controlled. The method for producing an alloyed hot-dip galvanized steel sheet according to claim 5, wherein the steel sheet contains, by mass%, C: 0.0001 to 0.3%, Si: 0.01 to 4%, and Mn: 0.01 to 2%, P: 0.002 to 0.2%, and Al: 0.0001 to 4%.